Generally, in the aeronautics field, after repair of a part with a view to reuse, a certain number of tests are necessary in order to mechanically validate the part to guarantee a satisfactory lifetime after repair.
A test generally carried out is that of the vibratory fatigue test, better known under the acronym HCF designating: “High Cycle Fatigue”. These tests make it possible to standardise a repair in the sense where it may then be industrialised on a large number of structurally identical parts by the same repair method while guaranteeing a satisfactory lifetime of the repaired parts.
It is thus necessary to characterise or validate the abatement of the material in HCF after repair. A known method is that called the “staircase” method, which may for example be carried out on repaired turbine blades.
However, the method comprises several drawbacks.
In the first place, the method requires loading the repaired blade according to a particular mode in order to be able to render the repaired zone critical, FIG. 1 illustrates a blade 1 having a damaged leading edge 2. The leading edge is intended to be repaired by a repair part, also called “patch” more commonly in the professional terminology. The patch is generally welded onto a surface of the part corresponding to the damaged portion. In the case of a blade, it is most often the leading edge. The repair forms at its limit a weld line 3. The test method enables stresses to be generated on the blade so as to fatigue test the repaired blade. The zones 4, 5 and 6 represent zones that deform under the effect of bendings applied to the part under vibration generating high levels of stresses. The vibratory fatigue tests are carried out at a mechanical resonance frequency of the part.
The zone 4 represents strips forming arcs of circles of which the strips the closest to the leading edge are the most deformed.
A first drawback is that this mode requires a high excitation energy to generate a sufficient level of stress in the repaired zone, which can with difficulty be generated by test means such as a vibrating table used conventionally.
The existing test method, which consists in determining the endurance limit in the weld, in other words at the level of the repair, and thus the determination of the abatement, does not make it possible to validate a maximum abatement level because the loading levels in the repaired zone are limited by the test means.
Moreover, a second drawback is the duration of a test, in other words the duration during which the part has to be vibrated at the resonance frequency of the mode in question, which is relatively long. As an example, for the first bending mode (1F) at 58 Hz, this duration is around 48 hours. Yet a particularity of the “staircase” method is that it comprises a large number of vibratory cycles for each part, by way of example at least 10,000,000 vibratory cycles for each part in current usage.
During the validation of a repair of blades by welding of a long patch on a leading edge, the cost and the duration of the test can become potentially problematic to validate the repair.